LG inverter

[sabre504]

Hi have an LG 42 " LCD and the inverter had gone so i bought another inverter (second hand )
It works but the screen pulses and the inverter buzz's

I have tested all the transformers and have found that 2 of the transformers are reading different from the other 10

1st pic i have under lined the 2 (i call them 6 and 7)

Test i did analogue meter set to 1K

1 red probe
2= 45
3 = 0.3
4 = 55
Had the same reading on 1,2,3,4,5,8,9,10,11and 12

Trans 6 and 7
1 red probe
2 = 140
3 = 0.3
4 = 180

Power supply putting out 24.2 volts steady on all 4 pins

So i am right in thinking that the 2 transformers are failing
as the are seprate lamps 6 on lamp 3 7 on lamp 4

Any input welcome

[Dgtech]

Re: LG inverter

I'd look at the circuitry leading up to that transformer. Look at the driver chip(s) or the mosfets. I'd point them out but the picture of the whole board is fuzzy. Post a clear picture of the whole board if you can.

[PlainBill]

Re: LG inverter

You aren't doing the measurement correctly. The pins you have labeled 1 and 3 are the ends of the primary. There is no point in measuring them, the resistance will be so low that your meter can't resolve the differences. The pins you have labeled 2 and 4 are the secondary. Measure the resistance across them for each of the transformers.

Interestingly, it looks like the primaries are in series. It would be nice to have a better resolution over-all picture.

Also, note that the manufacturer has already labeled the pins 1-6 (but skipped 3), and has numbered the transformers.

PlainBill

[sabre504]

Re: LG inverter

Hi again right have better pics but had to split in 2 as it was too long a board to get decent pic all in one .

I sussed i had measured wrong when i took one of to test off board and also found it had a small heat mark under the transformer so took the other off too and same mark underneath.
So have taken 2 transformers off the other board i had soldered them on and remeasured ,all read the same now so resoldered all pins on all transformers .

Also noted some poor solder on the transistors (??) i think they are or maybe drivers or mosfets and the one i have ringed in yellow measures different from the rest.
Meter on 1K
probe on each leg measured 6
one i have ringed measured 0 open so am going to change it with one from the other board i probably not measured it right but def something wrong with it .

Will resolder all those too and report back
Thanks for the input from you both

[Village Electronics]

Re: LG inverter

Hi have an LG 42 " LCD and the inverter had gone so i bought another inverter (second hand )
It works but the screen pulses and the inverter buzz's

I have tested all the transformers and have found that 2 of the transformers are reading different from the other 10

1st pic i have under lined the 2 (i call them 6 and 7)

Test i did analogue meter set to 1K

1 red probe
2= 45
3 = 0.3
4 = 55
Had the same reading on 1,2,3,4,5,8,9,10,11and 12

Trans 6 and 7
1 red probe
2 = 140
3 = 0.3
4 = 180

Power supply putting out 24.2 volts steady on all 4 pins

So i am right in thinking that the 2 transformers are failing
as the are seprate lamps 6 on lamp 3 7 on lamp 4

Any input welcome

You say that "it works but"..... To me that indicates the panel is lighting.

Based on that assumption....

I have found that an effective way to get a handle on inverter board transformers is to put the plastic shielded end of my scope probe against the transformers one by one. The signal induced should be approximately the same on each transformer. If one is much different than the others that is where you have to concentrate.

To isolate a single transformer that you suspect: Assuming you have no sub handy, simply swap the suspect transformer with another. If the the bad indication swaps then suspect the transformer. If it does not swap suspect the circuit.

[sabre504]

Re: LG inverter

Sorry i wasnt clear about the fault.

Fault being the screen had picture and sound but would appear to flicker or pulse as i put was giving the impression that the brightness was ramping up and down but i assume it was lamps 3 and 4 flickering .
Ihave swapped the 2 transformers and the other part resoldered everything i could and now appears to be working .
My 3 year old is testing it now with a marathon of Toy story

Thanks again all for the help and advice ,will just have to see how it goes and if it lasts

[Dgtech]

Re: LG inverter

Good job! Keep that other board handy though, just in case.

[sabre504]

Re: LG inverter

I will you never when someone else may need a transformer

Thanks again all

[sabre504]

Re: LG inverter

Still going strong ,
have got the brightness down to 25 and backlight on 50 and still clear and bright